Some manufacturers offer a second repeater head that displays the same information as the primary head but these systems are expensive. (Raytheon and B&G are 2 off the top of my head.)
The cheaper alternative is to fit a second set with transducer, if you are just using it to see when you are coming over the wreck a cheap set about £100 will be fine. There is a problem with this though. You have to make sure the two sets use different frequencies or they will interfere with each other, or you can switch off the set you are not using. Most sounders use a frequency of 200Hz but there are some that have dual frequency 50 Hz and 200Hz, these tend to be the higher price range. You could use a pair of these with one set at 50Hz and the other on 200Hz. The one set you use on 50Hz will show less detail though.
The cheapest way to go it to use a Humminbird along with another make. Some Humminbirds operate at 180Hz and will not interfere with set operating on 200Hz so you can run the 2 together. I have this set up on my boat with a good Lowrance in the wheelhouse and a base model Humminbird on deck.